1. Field of the Invention
This invention relates to a moisture-curing hot-melt adhesive, its production and its use in the shoe industry.
2. Discussion of Related Art
A "moisture-curing polyurethane hot-melt adhesive" is understood to be an adhesive largely free of solvents and having urethane groups, which is solid at room temperature and following application in the form of its melt physically binds not only by cooling but also by chemical reaction of still present isocyanate groups with moisture. It is only subsequent to such chemical curing with increase in molecule size that the adhesive receives its final characteristics.
Such moisture-curing polyurethane hot-melt adhesives are known for various substrates:
EP 0,421,154 describes a rapidly crystallizing polyurethane system containing isocyanate groups which is based on a polyester diol A having a melting point of from 30 to 60.degree. C. and a polyester diol B having a melting point of from 65 to 150.degree. C. and a polyisocyanate D. In addition, it may contain a diol C having a molecular weight of &lt;400 as a chain extender. The polyurethane system may be directly prepared from these components in a single step but multiple steps are possible as well. The thus obtained polyurethane system is partially crystalline and is intended to be used as an assembly adhesive but specifically, as a book-binding adhesive.
DE 3,236,313 describes a hot-melt adhesive which consists of from 20 to 90% by weight of a prepolymeric isocyanate and from 5 to 50% by weight of a low molecular weight ketone resin and, in addition, may contain from 0 to 75% by weight of a thermoplastic polyurethane. The prepolymeric isocyanate is a reactive polyurethane prepolymer of
1. an aromatic diisocyanate and/or PA1 2. a prepolymer of said isocyanate, and of PA1 3. a polyether or a polyester having OH groups, and PA1 4. a short-chain diol. PA1 A) at least one polyurethane prepolymer of PA1 B) optional additives such as PA1 a) aliphatic hydroxycarboxylic acids, or from PA1 b) aliphatic dicarboxylic acids having from 6 to 12 C atoms and--particularly even-numbered--diols having from 4 to 8 C atoms. PA1 1. adipic acid, isophthalic acid, phthalic acid, and butanediol; PA1 2. adipic acid, phthalic acid and hexanediol; PA1 3. adipic acid, isophthalic acid, phthalic acid, ethylene glycol, neopentylglycol, and 3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethyl propanoate; and PA1 4. adipic acid, phthalic acid, neopentylglycol, and ethylene glycol. PA1 a) from 15 to 35% by weight of polyisocyanate, PA1 b) from 10 to 70, in particular, from 15 to 35% by weight of a polyalkylene glycol, PA1 c) from 5 to 65, in particular, from 30 to 50% by weight of a polyester glycol, PA1 d) from 0 to 15, in particular, from 3 to 10% by weight of a resin, and PA1 e) from 0 to 0.5, in particular, from 0.01 to 0.1% by weight of a stabilizer. PA1 1. optionally, pretreatment of sole and/or shoe upper material, for example, by roughing, priming, halogenating, solvent-wiping, etc.; PA1 2. coating one layer of melt adhesive at 110 to 180.degree. C. on at least one of the surfaces to be attached, wherein the other one is not required to contain any adhesive or a solvent adhesive or a dispersion adhesive; PA1 3. pressing together the surfaces to be bonded; and PA1 4. optionally, cooling the surfaces to be bonded.
In the only preparative example, there is added in addition to 516 g of a polyester diol another 74 g of tripropylene glycol as a chain extender. The prepolymer is prepared from the components in a single step and has a viscosity of from 50 to 40,000 Pa.s at 20.degree. C. The hot-melt adhesive is suitable for bonding thermoplastic and thermo setting adhesives, foamed material, lacquered surfaces, wood and wooden materials, paper, leather, artificial leather, rubber, textiles, non-woven materials, and metals.
EP 0,340,906 describes a polyurethane hot-melt adhesive containing a mixture of at least two amorphous polyurethane prepolymers conferring different glass transition temperatures to the hot-melt adhesive which are above and below room temperature, respectively. The prepolymers are prepared from polyisocyanates and various diols, namely, preferably polyester diols on the one hand and, preferably polyester, polyether and other polymer diols on the other hand. The hot-melt adhesive is prepared by either mixing the individually prepared prepolymers or by preparing the second prepolymer in the first. As a rule, there are obtained opaque hot-melt adhesives at room temperature which, according to the example, have a viscosity of 91 Pa.s at 130.degree. C. The polyurethane hot-melt adhesive is used for wood bonding.
EP 0,369,607 describes a moisture-curing polyurethane hot-melt adhesive containing at least two polyurethane prepolymers, wherein the first prepolymer has a glass transition temperature of above room temperature and the second prepolymer has a glass transition temperature below same. The hot-melt adhesive is prepared from polyisocyanate and various diols, namely, from a polyether diol on the one hand, and a polyether, polyester or polybutadiene diol on the other hand. Example III contains 8.5% by weight of a polyester diol. It is indicated that the polyurethane hot-melt adhesive may be prepared in a single or in multiple steps. As a rule, it is transparent at room temperature and, according to Example 1, has only one single glass transition temperature. According to the Examples, its viscosity is between 6.8 and 20 Pa.s at 130.degree. C. The hot-melt adhesive has been used for bonding wood, plastics and textiles.
EP 0,472,278 describes an adhesive of a polyalkylene ether diol, a polyalkylene ether triol, a polyester polyol, and an aliphatic isocyanate. It is a moisture-curing hot-melt adhesive for textiles, wood, metal, leather, and plastics.
EP 0,242,676 describes a process for preparing polyurethanes having free isocyanate groups, which are soluble in aromatics, suitable as varnish binders and are obtained by reacting diisocyanates, macropolyols having a molecular weight of from 500 to 10,000 and low molecular weight polyols having a molecular weight of from 62 to 499. These latter are strongly branched diols such as 2,2,4-trimethylpentane-1,3-diol, the hydroxyl groups of which may further be etherified by two ethoxy or propoxy units. The polyurethanes are present as a solution and are used in moisture-curing one-component varnishes.
EP 0,511,566 describes a moisture-curing polyurethane hot-melt adhesive consisting of a) a polyol component which is liquid at room temperature and has an average molecular weight of from 500 to 5,000, and b) a polyol component which is crystalline at room temperature and has a molecular weight of from 500 to 10,000, and a mixture of a polyisocyanate component having two differently reactive NCO groups and a diisocyanate component having an NCO reactivity to hydroxy groups which is greater than that of the less reactive NCO groups of the polyisocyanate component with two differently reactive NCO groups. Preferably, the hot-melt adhesives are used for bonding glass and glass-plastic composites.
EP 0,455,400 describes a moisture-curing polyurethane hot-melt adhesive essentially consisting of the following mixture:
a) 10 to 90 parts by weight of a first polyurethane prepolymer of polyhexamethylene adipate, and
b) 90 to 10 parts by weight of a second polyurethane prepolymer of polytetramethylene glycol having a molecular weight Mn of at least 500.
This hot-melt adhesive is suitable for bonding certain synthetic polymers, wood, paper, leather, rubber, textiles, including non-wovens, and metals.
Furthermore, it is known to use moisture-curing polyurethane hot-melt adhesives in the shoe industry.
Thus, EP 0,125,008 describes a process for bonding a shoe sole to a shoe upper. The polyurethane is prepared from a diisocyanate, a hydroxypolyester and a monofunctional reactant, wherein the monofunctional reactant preferably is an aliphatic alcohol having from 4 to 14 carbon atoms and a boiling point of above 100.degree. C. Following adhesive application and prior to actual bonding by heating and pressing, the adhesive is subjected to humidity to extend the chains. This requires from 10 to 15 minutes at 80 to 110.degree. C. and a moisture corresponding to a dew point of from 40 to 60.degree. C. and subsequent treatment at atmospheric humidity for 10 minutes to 18 hours.
Another process for attaching a sole to a shoe is described in EP 0,223,562. First, a moisture-curing solvent-free hot-melt adhesive is applied to the sole and/or the shoe upper material, then the adhesive coating is subjected to hot-curing with moisture until eventually, sole and shoe upper material are pressed together.
Likewise, DE 2,609,266 describes a process for bonding shoe soles to shoe uppers. The hot-melt adhesive is prepared by reacting a diisocyanate with a polymeric polyol having a crystalline melting point of from 40 to 90.degree. C. Following application of the adhesive, the latter is, e.g., contacted with water, and then the thus treated coating in a heat-softened condition is pressed to a surface compatible therewith.
However, known moisture-curing polyurethane hot-melt adhesives have severe drawbacks when applied in the shoe industry which oppose wide range economic application:
The adhesive, if ever possible, should not contain any toxic substances, in particular, no solvents.
Coating should be possible at a temperature not too high for the shoe materials.
The shoe materials to be bound are predominantly leather and rubber.
The adhesive should have suitable flow behavior and good wetting at any application temperature so that it can be coated without problems.
The adhesive bond should have sufficient initial strength for further processing. Specifically, this holds for creep resistance and initial strength, where curing should be effected in an adequate period of time and at ambient conditions, if possible. That is, the adhesive should be cured within a few minutes to such extent that its surface is no longer tacky so that subsequent to cooling, coated soles and shoe uppers may be easily handled and stored without sticking to each other. Moreover, the bond between sole and shoe upper should initially be so strong that it will come loose neither immediately nor subsequent to storage due to small but steady forces. Hence, sufficient initial strength and above all, high creep resistance must be achieved within the time intervals of shoe production. Curing is desired to occur at normal ambient conditions of sole storage (20.+-.5.degree. C., ca. 10% relative humidity) within 24 hours, if possible, but within 7 days at latest.
In use, the adhesive bond should be sufficiently strong and flexible. The cold temperature flexibility is of particular significance.
Thus, the object of the invention is to provide the shoe industry with a moisture-curing polyurethane hot-melt adhesive coatable by machines integrated in the shoe production line not including any previous crosslinking by steam and drying in a climatic channel. To this end, the adhesive is required, in particular, to gain high creep resistance with sufficient initial strength and final strength in an adequate period of time. The other characteristics of processing and use should not be deteriorated. In particular, this holds for non-toxicity and cold temperature flexibility.